Absorption heat pumps, chillers, refrigerators and air conditioners (hereafter, collectively referred to as xe2x80x9cheat pumpsxe2x80x9d) use an ammonia/water working fluid, as well as other types, to transfer heat. Absorption heat pumps offer high heating and cooling efficiencies without the use of refrigerants that are harmful to the environment. The structure and functioning of an absorption heat pump are further described in U.S. Pat. No. 5,811,026, which is herein incorporated by reference in its entirety.
To compete effectively in the marketplace, advanced absorption systems, such as the Generator-Absorber heat exchange (GAX) cycle heat pump, utilize low-cost materials of construction such as carbon steel, which lacks the corrosion resistance of more-costly alloys, such as stainless steel. The use of steel, especially mild steel, is advantageous because steel is easily formed and welded. Nevertheless, steel can be corroded by the highly-corrosive ammonia/water solution at the elevated temperatures typically required for highly-efficient thermodynamic cycles. Corrosion of the steel produces magnetite (Fe3O4) and hydrogen gas (H2) according to the following reaction:
3Fe+4H2Oxe2x86x92Fe3O4+4H2 
The magnetite coatings formed on the steel surface at elevated temperatures can become thicker with time, flake off and clog the tubes of the heat pump, while the formation of hydrogen and other non-condensible gases reduce the efficiency of the system. The formation of magnetite by a corrosion reaction decreases the structural integrity of pressure-retaining components of the heat pump. The loss of structural integrity is of particular concern in the high pressure components of the system, such as the desorber and condenser which operate at pressures as high as 300 psig to 450 psig.
Conventionally, a chemical inhibitor in the form of sodium chromate (Na2CrO4) or sodium dichromate (Na2Cr2O7) is added to the working fluid to inhibit the working fluid from reacting with the steel. Sodium chromate is effective for operating temperatures up to about 200xc2x0 C., which is 10xc2x0 C. or more below the typical peak solution temperature in the GAX cycle. It has been shown that sodium chromate (Na2CrO4) can react with ammonia at high temperatures to form N2 and NaOH according to the following reaction:
NH3+H2O+Na2CrO4xe2x86x92xc2xdN2+2NaOH+Cr(OH)3 
In this reaction, the chromium ion is reduced from a plus-6 to a plus-3 state. In the absence of an inhibitor, a much larger quantity of non-condensible gas, primarily hydrogen, is formed than in chromate-inhibited systems. Nitrogen gas is the primary non-condensible gas formed in chromate-inhibited systems. In chromate-inhibited systems, the rates of magnetite scale formation and chromate breakdown increase with increasing temperature. Further, the addition of chromate inhibitors imposes significant disadvantages due to the fact that chromium is highly toxic, having been identified as a human carcinogen by the International Agency for Research on Cancer. In addition, chromium pollutants present substantial environmental hazards. As a result, their use is being phased out in many localities.
Applicants have found that rare earth metal salts can be substituted for chromates as chemical inhibitors in ammonia/water heat pumps avoiding the health and environmental risks, reactivity and temperature limitations of sodium chromates.
A method of Applicants"" invention includes the step of introducing a rare earth metal salt to the ammonia/water working fluid in a heat pump to inhibit corrosion of the heat pump""s steel surfaces. The rare earth metal salt can be a cerium salt of the cerous salt form, preferably, cerium nitrate. The concentration of the rare earth metal salt can be about 10 mM to about 350 mM. Preferred embodiments of the method of this invention include a dual protection method of pre-treating the steel surface with a cerium oxide/hydroxide layer to prevent both corrosion of the metal and ammonia dissociation, and adding rare earth metal salts to the solution to act as a corrosion inhibitor. In addition to acting as a corrosion protective layer, the cerated coating insulates the metal surface, preventing electrochemical reactions, which can result in generation of non-condensible gases.
An apparatus of this invention includes a heat pump having a steel housing and an ammonia/water working fluid contained in the steel housing. In preferred embodiments, the heat pump""s corrosion resistance is enhanced by cerating the steel surfaces that will be exposed to the ammonia/water working fluid.
The methods and apparatus of this invention offer a number of advantages. For example, the use of a rare earth metal salt, such as cerium nitrate, provides a degree of corrosion inhibition similar to that of Cr6+without incurring the health and environmental risks that are posed by chromates. Further, the use of a rare earth metal salt in accordance with this invention also reduces the amount of non-condensible gas generated in the heat pump. Further still, a smaller quantity of cerium nitrate appears to be required to match the inhibition performance of chromates, therefore providing a potential for cost savings, as well. Finally, cerium salts are very inexpensive.